1. Field of the Invention
This invention relates to proton conducting polymer electrolytes used in solid polymer electrolyte fuel cells. In particular, it relates to sulfonated poly(phenylene) based copolymer electrolytes.
2. Description of the Related Art
Proton exchange membrane fuel cells (PEMFCs) convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). MEA durability is one of the most important issues for the development of fuel cell systems in either stationary or transportation applications. For automotive applications for instance, an MEA may be required to demonstrate durability of about 6,000 hours.
The membrane electrolyte serves as a separator to prevent mixing of reactant gases as well as an electrolyte for transporting protons from anode to cathode. Perfluorosulfonic acid (PFSA) ionomer, e.g., Nafion®, has historically been the material of choice and the technology standard for both membranes and for ionomer employed in the catalyst layers of an MEA. Nafion® consists of a perfluorinated backbone that bears pendent vinyl ether side chains, terminating with SO3H. Nafion® membranes show good operation under normal operating conditions but have several disadvantages. They are expensive, limited to operation at relatively low temperatures, and offer a poor permeance barrier to the hydrogen and oxygen reactants, which reduces the durability of a fuel cell stack and lowers fuel efficiency and the driving range of fuel cell vehicles. Further, Nafion® can release fluorine compounds upon decomposition, which can cause catalyst dissolution and raise environmental concerns. When used in a catalyst layer, the strong acidity of Nafion® can also accelerate degradation of the catalyst.
Accordingly, there is a desire to find an alternative ionomer for Nafion®. Several classes of hydrocarbon or semi-fluorinated polymers are under intense investigation. These include poly(ether arylenes), polyimides, polyphosphazenes, radiation-grafted polystyrene, organic-inorganic composites and hybrids, polystyrene di- and tri-block copolymers, and acid-complexes of basic polymers. However, most hydrocarbon membranes cannot meet the durability requirement of automotive fuel cells due to the presence of weak bonds in the ionomer chains. For instance, α-hydrogen of polystyrene is not stable under free radical attack, while ether bonds and polyimide structure are hydrolytically unstable (T. J. Peckham et al, Proton Exchange Membranes, in Proton Exchange Membrane Fuel Cells: Materials Properties and Performance, Editors: David P. Wilkinson et al, CRC Press, 2009, 107-189). Furthermore, most hydrocarbon membranes have insufficient performance under low RH (˜30%) for automotive fuel cells. JSR Corporation and Honda Motor Co., Ltd developed an aromatic hydrocarbon PEM (K. Goto, et al Polymer Journal, 2009, 41(2): 95104; U.S. Pat. No. 7,449,132, US20100174042, U.S. Pat. No. 7,893,303). This aromatic polymer electrolyte comprised a hydrophilic block of sulfonated poly(benzophenone) and a hydrophobic block of poly(ether sulfone) or poly(ether ketone). In addition, the aromatic polymer electrolyte comprises ether bonds, either in the main chain of the hydrophobic block or in both side chains of the hydrophilic block and the main chain of the hydrophobic block.
Recently however, it was reported that hydrocarbon membranes with dense sulfonic acid groups exhibit higher performance than PFSA membrane at high temperature (S. Tian et al, Macromolecules, 2009, 42, 1153-1160; K. Matsumoto et al, Macromolecules, 2009, 42, 1161-1166). While these hydrocarbon membranes with dense sulfonic acid groups were synthesized by condensation polymerization of dichlorodiphenylsulfone or difluorobenzophenone with dihydroxy monomers followed by post-sulfonation, limited durability of these membranes in fuel cells can be expected due to the existence of ether bonds in the ionomer chains.
There remains a continuing need for improved hydrocarbon ionomer electrolytes for solid polymer electrolyte fuel cells and, in particular, for electrolytes exhibiting good performance and durability characteristics. This invention fulfills these needs and provides further related advantages.